The invention is based on a priority application EP 05291798.6 which is hereby incorporated by reference.
The invention relates to a method for determining crosstalk coupling between a plurality of transmission lines in digital data transmission systems, in particular DSL networks.
The invention also relates to a crosstalk determining unit for integration in or for connection to an access node of a digital data transmission system, in particular a DSL network, with a plurality of transmission lines coupled to the access node.
Furthermore, the invention relates to a digital data transmission system, in particular in the form of a DSL network, and to a computer program product for determining crosstalk coupling in digital data transmission systems, in particular DSL networks, with a plurality of transmission lines.
Crosstalk remains one of the major limiting factors for Digital Subscriber Lines (DSL) transmission, as it effectively limits the obtainable DSL bitrate (for a given loop length) or the DSL reach at a guaranteed minimum bitrate. Furthermore, crosstalk can cause errors in transmission, service interruption, and the need for time consuming re-initializations and re-synchronizations. As a consequence crosstalk plays a major role in DSL service deployment. Additionally, knowledge of crosstalk coupling is equally valuable for other DSL deployment scenarios, e.g. service upgrading, fault diagnosis, etc. For all DSL deployment phases (such as prequalification or in-service optimisation), solutions are needed that help to estimate the impact of crosstalk, or even to minimise it (such as dynamic spectrum management DSM). For instance, when DSL lines are limited by far-end crosstalk (FEXT), FEXT needs to be mitigated in order to upgrade DSL users to higher bit rates. Alternatively, for a given service FEXT mitigation can largely extend the reach of DSL. The same holds also for near-end crosstalk (NEXT).
An important issue in this context is to determine which transmission lines (hereinafter also referred to as “lines”) are crosstalk-affecting one another. Typically a cable leaving a Digital Subscriber Line Access Multiplexer (DSLAM) contains thousands of lines. Crosstalk coupling is present between all line couples, but the crosstalk coupling is not equally strong for each line couple. Especially when lines inside a cable are grouped in binders, crosstalk coupling between lines in the same binder is on average higher than crosstalk between lines in separate binders. In order to optimise the performance of a DSL network and to limit its complexity, e.g. by means of DSM algorithms, an important point is to know which lines are contained in one binder. Although databases exist from which one may deduce which lines are in the same cable or cable binder, these databases are not always updated, so that their reliability is estimated to be between 60 and 80%. Furthermore, from field tests, it is well-known that there is a lot of variation of crosstalk coupling between line couples—up to 20 dB—which cannot be accurately determined solely on the basis of the mechanical design of the loop plant.
A first kind of prior art solutions for determining crosstalk levels rely on worst-case assumptions: Since the crosstalk coupling functions and the users that share the same binder are generally unknown, DSM algorithms have to resort to worst case crosstalk coupling functions, which are often overly conservative. An alternative prior art approach as described in European patent application 04 292 070.2 incorporated herein by reference includes Virtual Binder Identification through polling: This technique allows to detect which lines generate the highest amount of crosstalk towards a given transmission line of interest. It consists in continuously monitoring the on/off status and/or the noise margin of modems on different transmission lines. If a given modem is switched on and, as a consequence, a second modem switches off or experiences a loss in noise margin, the first modem/line can be identified as a dominant crosstalker with respect to the second modem/line. At the same time a crosstalk coupling constant can be estimated.
The aforementioned prior art approaches suffer from inherent disadvantages: For instance, the worst case assumption leads to overly conservative designs, wherein two users which share the same binder without affecting each other, will transmit with a “worst case” power spectral density (PSD), instead of transmitting at full power. Furthermore, the resulting spectrum management is static in nature and thus does not change when new users become active or when the loop topology is changed. Concerning Virtual Binder Identification through polling, polling the on/off status and/or the noise margin of each single line is highly complex and infeasible in practice since on/off switching of modems is controlled by the end user and not by the central agent. Hence, the agent would have to monitor all lines (at the same time) to see whether or not the status of a particular modem changes. A DSLAM, however, can only process a small number of simple network management protocol (SNMP) commands per minute, which limits the number of lines that can be monitored at a given time. Still another disadvantage concerns the uniqueness of the polling result: If two modems (“disturbers”) switch on at essentially the same time, and two other modems (“victims”) have to retrain or find their noise margin reduced as a result of this, it is impossible to determine which victim is affected by which disturber. Hence, the polling process of crosstalk detection is not unique, which may lead to incorrect spectrum management.